Speckle patterns are created when coherent light is reflected from a target surface. If the target surface or coherent light source moves laterally, the associated speckle pattern also moves. This provides the operational basis for speckle based navigation systems. Typically, these speckle based navigation systems involve navigation in two dimensions, see for example, Schnell, Piot and Daendliker, “Detection of movement with laser speckle patterns: statistical properties”, JOSA A, vol. 15, 1, pp. 207–216, 1998. Speckle patterns are interference patterns emitted from target surfaces illuminated by coherent light. If the target surface moves, the associated speckle pattern is moved as well. This physical phenomenon provides the basis for speckle based navigation sensors. Typically, speckle based navigation sensors include a laser light source, optical components and a photodetector. The speckle pattern consists of speckle “beams” that are emitted nearly isotropically from the illuminated target surface. The characteristic of nearly isotropic emission means that for lower levels of illumination the speckle flux in any one direction is relatively low and reduces the speckle sensed by the photodetector. Hence, sensors in speckle based navigation typically suffer from low collection efficiency.
Some speckle based navigation systems, see for example, Ogita, Ueda and Yamazaki, “Optical three-dimensional displacement meter”, Proceedings of the SPIE International Conference on Speckle, vol. 556, pp. 139–145, 1985, use speckle for two dimensional navigation and either flux variation or interference fringe counting to provide for navigation in the third dimension. In the first approach, increasing the separation between the sensor and the target surface reduces the flux reaching the sensor according to the inverse square law. Monitoring the flux variation enables motion detection in the third dimension, the direction normal to the target surface. The amount of flux reflected from the target surface is dependent on the reflection and scattering properties of the target surface. This introduces a target surface dependence into this approach.
In the second approach, the target surface dependence is eliminated. However, the second approach is relatively involved and typically requires multiple optical components. Interference fringes are generated from the interference of a first beam from a coherent light source reflected internally in the sensor with a second beam from the coherent light source reflected from the target surface. The interference fringes are counted as the sensor separation from the target surface is changed. Each fringe represents a change of separation between the sensor and the target surface of one wavelength. Typically, a complex optical path with multiple beam splitters is required to split the coherent light beam into a first and second beam while maintaining adequate coherence correlation between the first and second beams.
Navigation capability in the third dimension is useful, for example, for use with writing capture systems for whiteboard type surfaces.